1. Field of the Invention
This invention relates to a semiconductor light emitting device, and more precisely to a visible light emitting element (semiconductor laser, light emitting diode, etc.), in particular to the structure of a light emitting diode which has an AlGaInP active layer formed by means of epitaxial growth on a GaAs substrate.
2. Prior Art
A light emitting diode in which a light emitting layer portion consisting of an AlGaInP double heterojunction structure is known to have a very high quantum efficiency, and has been put into practical use as a high brightness visible light emitting diode. The light emitting layer portion comprises an AlGaInP active layer and first and second AlGaInP cladding layers which sandwich the active layer. These AlGalnP layers have different composition ratios and are grown on a GaAs substrate while lattice match is maintained.
However, the energy of photons emitted from the AlGaInP active layer is larger than the band gap of GaAs which constitutes the substrate, hence the GaAs substrate acts as an absorption layer for the emitted light. Therefore, the light emitted toward the GaAs substrate is absorbed by the GaAs substrate, leaving only the light emitted toward a light extracting side to contribute to the brightness of the light emitting diode, thus resulting in inefficient brightness.
For the purpose of reducing absorption of the emitted light by the GaAs substrate, there is a method in which a light reflecting layer comprising alternately laminated layers with different refractive indices is provided between the GaAs substrate and the light emitting layer portion so that the light emitted toward the GaAs substrate is reflected by the light reflecting layer toward the light extracting side, so as to improve the light extraction efficiency. The light reflecting layer is generally formed by alternately laminating a large number of layers with a high refractive index and layers with a low refractive index wherein the optical thickness of each layer is approximately 1/4 of the wavelength of the emitted light. AlGaAs, AlGaInP, or a combination of these materials have been used for the light reflecting layers.
A light reflecting layer using Al.sub.w Ga.sub.1-w As is most widely used partly because Al.sub.w Ga.sub.1-w As is in approximate lattice match with GaAs. For example, a light reflecting layer formed by alternately laminating Al.sub.0.6 Ga.sub.0.4 As layers and Al.sub.0.8 Ga.sub.0.2 As layers is used in particular.
FIG. 3 shows a cross-sectional structure of a conventional light emitting diode which has a light reflecting layer using Al.sub.w Ga.sub.1-w As. In this light emitting diode, a light reflecting layer 11, a light emitting layer portion 15 and a p-type current spreading layer 16 are formed one after another on an n-type GaAs substrate 10. The light emitting layer portion 15 has a double heterojunction structure comprising an n-type AlGaInP cladding layer 12, an AlGaInP active layer 13 and a p-type AlGaInP cladding layer 14 layered one after another. The composition ratio in said active layer 13 is determined according to a desired emission wavelength. An example would be (Al.sub.y Ga.sub.1-y).sub.0.51 In.sub.0.49 P (where 0.ltoreq.y.ltoreq.0.7). The light reflecting layer 11 comprises a large number of alternately laminated n-type Al.sub.0.6 Ga.sub.0.4 As layers 11a and n-type Al.sub.0.8 Ga.sub.0.2 As layers 11b, each having a thickness of approximately 400 .ANG.. The p-type current spreading layer 16 (for example, a p-type Al.sub.0.7 Ga.sub.0.3 As current spreading layer) is provided to effectively spread the current from a p-electrode (not shown in the figure) which is formed on said current spreading layer 16 into the entire area of the AlGaInP active layer 13 which is a light emitting layer, so that light emission can be more efficient.
As described above, Al.sub.0.6 Ga.sub.0.4 As and Al.sub.0.8 Ga.sub.0.2 As are in relatively good lattice match with GaAs. However, there is approximately 0.1% lattice mismatch at room temperature, causing an internal stress in the light reflecting layer. This leads to deterioration of the light emitting characteristics when the light emitting diode is used for light emission by supplying an electricity over an extended time.
When AlGaInP is used for the light reflecting layer, the lattice mismatch with GaAs at room temperature can be made very small by controlling the mixed crystal composition, particularly the In composition. Therefore, it is possible to form a light reflecting layer with a considerable reflectivity and no internal stress, by optimizing the compositions of the alternately laminated AlGaInP layers. However, since it is difficult to grow an AlGaInP layer with a total thickness of over 3-4 micrometers on a GaAs substrate without compromising the crystalline quality, it is preferable not to use this material for the light reflecting layer.